Nuclear still shines in the UK

On Friday 26 May more than 60% of electricity generation in the UK came from low carbon sources. By far the largest single source was nuclear energy. However, that might not be the impression gained from news reports that day, such as the FT's Solar outshines nuclear as spring sun boosts UK output and BBC News's UK achieves solar power record as temperatures soar. This briefing examines that facts behind this. It also examines what generation option has the greatest potential for securing further greenhouse gas emissions reductions.

Share of UK generation, main generation sources 26 May

Many of the news reports that day focused on the peak of solar generation, around midday when the sun shone strongest. At this time about 24% of electricity generation in the UK was supplied by solar, a fraction more than the 23% supplied by nuclear. But, as the chart below shows, using data from Gridwatch, solar generation only exceeded nuclear generation for around three hours. Away from this peak solar output declines rapidly, whereas nuclear generation remains near-constant throughout the day.

All UK sources of electricity, 26 May

This chart also shows the contribution to UK electricity supplies from four interconnectors and some minor generation sources, which, for clarity, are omitted from the other charts. It should be noted that the French interconnector, supplying 2 GW of electricity for much of the day, is supplied predominantly from nuclear energy.

Just the nuclear electricity generated in the UK supplied more than two and half times as much electricity as solar, which itself was just beaten by wind generation over the whole day.

What would be the most effective option for reducing emissions further?

Solar generation made a valuable contribution to reducing emissions when it broke its own record for output on 26 May. By generating extra electricity during the daytime peak in demand it reduced the need for generation from gas, traditionally used for peaking demand. With coal generation now a very minor part of the generation mix in the UK switching to gas is no longer an effective option for reducing greenhouse gas emissions. So to reduce greenhouse gas emissions the UK needs to reduce its use of gas.

The chart below shows actual generation over the whole of 26 May. Nuclear generates an almost constant 8 GW over the entire 24 hours, while solar reaches its peak during the middle of the day. There are further low carbon contributions from wind, hydro and biomass.

UK generation, 26 May (MW)

To explore what would be the most effective way of reducing greenhouse gas emissions further let's examine what would be the effect of doubling the contribution of nuclear or solar, which both supplied around 8 GW of electricity at their peak.

Double Solar Scenario (MW)

If solar capacity were doubled solar generation would displace more of the midday peak gas-fired generation. However, this additional displacement still reduces to zero away from this peak and there is no reduction in the requirements for gas throughout the night. Note that for these comparisons it is assumed coal is phased out and replaced by gas, if required.

Double Nuclear Scenario (MW)

If the capacity of nuclear is doubled the result is very different.The additional nuclear generation is able to displace CCGT gas over the whole 24 hours. The constant generation of nuclear throughout the day is more effective in reducing the need for gas.

The greater effectiveness of a doubling of nuclear capacity compared to solar is shown below. With double nuclear capacity the share of gas is reduced from 38% to 12%. If solar capacity is doubled the share of gas is only reduced to 30%.

Both nuclear and solar made valuable contributions to reducing greenhouse gas emissions in the UK on 26 May. Solar is particularly useful in meeting the additional requirements of the increased use of electricity in the daytime. But because nuclear generation displaces fossil generation over the whole 24 hours of the day a doubling the UK's capacity of nuclear generation would have been more effective in reducing greenhouse gas emission than the same increase in solar capacity.